Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital audio broadcasting (DAB), uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
One feature of digital transmission systems is the inherent ability to simultaneously transmit both digitized audio and data. Thus the technology also allows for wireless data services from AM and FM radio stations. The broadcast signals can include metadata, such as the artist, song title, or station call letters. Special messages about events, traffic, and weather can also be included. For example, traffic information, weather forecasts, news, and sports scores can all be scrolled across a radio receiver's display while the user listens to a radio station.
IBOC technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5A, in September 2005. NRSC-5, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/standards.asp. iBiquity's HD Radio technology is an implementation of the NRSC-5 IBOC standard. Further information regarding HD Radio technology can be found at www.hdradio.com and www.ibiquity.com.
Other types of digital radio broadcasting systems include satellite systems such as Satellite Digital Audio Radio Service (SDARS, e.g., XM Radio™, Sirius®), Digital Audio Radio Service (DARS, e.g., WorldSpace®), and terrestrial systems such as Digital Radio Mondiale (DRM), Eureka 147 (branded as DAB Digital Audio Broadcasting®), DAB Version 2, and FMeXtra®. As used herein, the phrase “digital radio broadcasting” encompasses digital audio and data broadcasting including in-band on-channel broadcasting, as well as other digital terrestrial broadcasting and satellite broadcasting.
Both AM and FM In-Band On-Channel (IBOC) broadcasting systems utilize a composite signal including an analog modulated carrier and a plurality of digitally modulated subcarriers. Program content (e.g., audio) can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers. The analog audio is delayed at the transmitter by a diversity delay.
In the absence of the digital audio signal (for example, when the channel is initially tuned) the analog AM or FM backup audio signal is fed to the audio output. When the digital audio signal becomes available, a blend function smoothly attenuates and eventually replaces the analog backup signal with the digital audio signal while blending in the digital audio signal such that the transition preserves some continuity of the audio program. Similar blending occurs during channel outages which corrupt the digital signal. In this case, the analog signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog when the digital corruption appears at the audio output. Corruption of the digital audio signal can be detected during the diversity delay time through cyclic redundancy check (CRC) error detection means, or other digital detection means in the audio decoder or receiver.
The concept of blending between the digital audio signal of an IBOC system and the analog audio signal has been previously described in, for example, U.S. Pat. Nos. 7,546,088; 6,178,317; 6,590,944; 6,735,257; 6,901,242; and 8,180,470, the disclosures of which are hereby incorporated by reference. The diversity delay and blend allow the receiver to fill in the digital audio gaps with analog audio when digital outages occur. The diversity delay ensures that the audio output has a reasonable quality when brief outages occur in a mobile environment (for example, when a mobile receiver passes under a bridge). This is because the time diversity causes the outages to affect different segments of the audio program for the digital and analog signals.
In the receiver, the analog and digital pathways may be separately, and thus asynchronously, processed. In a software implementation, for example, analog and digital demodulation processes may be treated as separate tasks using different software threads. Subsequent blending of the analog and digital signals requires that the signals be aligned in time before they are blended.